CPSC 121: Models of Computation 2010 Winter Term 2

CPSC 121: Models of Computation2010 Winter Term 2 Number Representation Steve Wolfman, based on notes by Patrice Belleville and others

Outline • Prereqs, Learning Goals, and Quiz Notes • Prelude: “Additive Inverse” • Problems and Discussion • Clock Arithmetic and Two’s Complement • 1/3 Scottish and Fractions in Binary • Programs and Numbers • Programs as Numbers • Next Lecture Notes

Learning Goals: Pre-Class By the start of class, you should be able to: • Convert positive numbers from decimal to binary and back. • Convert positive numbers from hexadecimal to binary and back. • Take the two’s complement of a binary number. • Convert signed (either positive or negative) numbers to binary and back. • Add binary numbers. Discuss point of learning goals.

Learning Goals: In-Class By the end of this unit, you should be able to: • Critique the choice of a digital representation scheme—including describing its strengths, weaknesses, and flaws (such as imprecise representation or overflow) —for a given type of data and purpose, such as (1) fixed-width binary numbers using a two’s complement scheme for signed integer arithmetic in computers or (2) hexadecimal for human inspection of raw binary data. Discuss point of learning goals.

Where We Are inThe Big Stories Theory Hardware How do we build devices to compute? Now: enabling our hardware to work with data that’s more meaningful to humans. (And once we have numbers, we can represent pictures, words, sounds, and everything else!) How do we model computational systems? Now: showing that our logical models can connect smoothly to models of number systems.

Prelude: Unsigned Integers We can choose any arrangement of Ts and Fs to represent numbers... But, we might as well choose something convenient. If we let F correspond to 0 and T to 1, then our representation is...

Prelude: Unsigned Integers ...base 2 numbers. When we represent negative numbers, the choice is also arbitrary, but may as well be convenient: • Just one representation for zero • Easy to tell negative from positive (or non-negative?) • Basic operations easy. But... What does it mean for basic operations to be “easy”?

Prelude: Additive Inverse The “additive inverse” of a number x is another number y such that x + y = 0. What is the additive inverse of 3? What is the additive inverse of -7? We want to be able to add signed binary numbers. We need x + -x to be 0. And, we want addition to be easy to implement.

Problem: Clock Arithmetic Problem: It’s 0500h. How many hours until midnight? Give an algorithm that requires a 24-hour clock, a level, and no arithmetic. A level is a carpentry tool, essentially a straightedge that indicates when it is either horizontal or vertical.

Clock Arithmetic 0500 is five hours from midnight. 1900 is five hours to midnight. 5 and 19 are “additive inverses” in clock arithmetic: 5 + 19 = 0. So are any other numbers that are “across the clock” from each other. That’s even true for 12. Its additive inverse is itself!

Clock Arithmetic Problem It’s 18 hundred. Without using numbers larger than 24 in your calculations, what time will it be 22*7 hours from now? (Don’t multiply 22 by 7!) • 0 hundred (midnight) • 4 hundred • 8 hundred • 14 hundred • None of these (Clock arithmetic is also known as modular arithmetic in mathematics.)

Clock Arithmetic: Food for Thought If we wanted negative numbers on the clock,we’d probably put them “across the clock” from the positives. After all, if 3 + 21 is already 0, why not put -3 where 21 usually goes? 0 -3 -6 -9 -12

Unsigned Binary Clock Here’s a 3-bit unsigned binary clock, numbered from 0 (000) to 7 (111). 000 111 001 110 010 101 011 100

Crossing the Clock To “cross the clock”, go as many ticks left from the top as you previously went right from the top. Here’s a clock labelled with 0 (000) to 3 (011) and -1 (111) to -4 (100). 000 111 001 110 010 101 011 100

Reminder:Two’s Complement Taking two’s complement of B = b1b2b3...bn: 111...1 - b1b2b3...bn ---------- x1x2x3...xn + 1 ---------- -B Flip the bits Add one

A Different View ofTwo’s Complement Taking two’s complement of B = b1b2b3...bn: 111...1 - b1b2b3...bn ---------- x1x2x3...xn + 1 ---------- -B 111...1 + 1 ---------- 1 000...0 - b1b2b3...bn ---------- -B Flip the bits Add one Or... Just subtract from 100...0

Two’s Complement vs. Crossing the Clock Two’s complement with k bits: Equivalent to subtracting from 100...000 with k 0s. “Crossing the clock” with k bits: Equivalent to subtracting from 100...000 with k 0s. 111...1 + 1 ---------- 1 000...0 - b1b2b3...bn ---------- -B 000 111 001 110 010 101 011 100 Two’s complement turns numbers into their “normal”, “cross-the-clock” additive inverses.

Problem: Why Two’s complement? Why make the negation of 010 be 110? • 010 + 110 already equals 0. • 010 + 110 equals 1000. • 110 is the easiest negation to calculate. • 110 isn’t being used for any other purpose. • If you invert the bits in 110 and add 1, you get 010. 000 111 001 110 010 101 011 100

Summary Questions (1 of 2) • With n bits, how many distinct values can we represent? • What’s the smallest/largest n-bit unsigned binary integer? • What’s the smallest/largest n-bit signed (using two’s complement) binary integer? • Why the “extra” negative number? • How many representations for each number do we have with unsigned/signed binary integers?

Summary Questions (2 of 2) • How do we tell if an unsigned binary integer is: negative, positive, zero? • How do we tell if a signed binary integer is: negative, positive, zero? • How do we negate a signed binary integer? • What one value does that negation fail (or at least do something weird) on for signed binary integers? • How do we calculate the decimal value of a positive signed binary integer? • How do we calculate the decimal value of a negative signed binary integer?

Problem: 1/3 Scottish Problem: Can you be 1/3 Scottish?

Problem: 1/3 Scottish Problem: Can you be 1/3 Scottish? To build a model, we must clearly specify the problem. Many problems admit multiple models that lead to fundamentally different results. We’re going to use the model of parentage “endowing” 50% of each parent’s “ish-ness”. That’s a coarse, even silly model. (By that model, none of us are Canadian, since humans did not originate in Canada.) Our model is handy for us, but it’s not necessarily what people’s identity is about!

Can you be one-third Scottish? Focus on Mom (and Mom’s Mom and so on). We’ll just make Dad “Scot” or “Not” as needed at each step. Mom:?? Dad:??

Can you be one-third Scottish? Mom:?? Dad:?? Mom:2/3 Dad:Not

Can you be one-third Scottish? Mom:?? Dad:?? Mom:1/3 Dad:Scot Mom:2/3 Dad:Not

And so on... Can you be one-third Scottish? Mom:1/3 Dad:Scot Dad:Not Mom:2/3 Mom:1/3 Dad:Scot What’s happening here? Mom:2/3 Dad:Not

0 . 0 1 0 1... Now, focus on Dad... We can represent fractions in binary by making “Scottish family trees”: Mom:1/3 Dad:Scot Mom:2/3 Dad:Not Mom:1/3 Dad:Scot Mom:2/3 Dad:Not

Here’s 0.375 in binary... 0 . 0 1 1 Mom:0.0 Dad:Scot Mom:0.5 Dad:Scot Mom:0.75 Dad:Not

Problem: 1/3 Scottish Which of the following numbers can be precisely represented with a finite number of digits/bits using a “decimal point”-style representation in base 10 but not base 2? • 1/9 • 1/8 • 1/7 • 1/6 • None of these.

So... Computers Can’t Represent 1/3? No! Using a different scheme (e.g., a rational number with a separate integer numerator and denominator), computers can perfectly represent 1/3! The point is: Representations that use a finite number of bits (all of them) have weaknesses. Know those weaknesses and their impact on your computations!

What Doesn’t Work isNot Always Obvious (1 of 2) Class Main { public static void main(String[] args) { // Let's add up 4 quarters. System.out.println("4 quarters gives us:"); System.out.println(0.25 + 0.25 + 0.25 + 0.25); // Let's do something a hundred times. int i = 100; do { // Make i one smaller. i--; } while (i > 0); System.out.println("Done!"); System.out.println("i ended up with the value: " + i); System.out.println("It went down by: " + (100 - i)); } }

What Doesn’t Work isNot Always Obvious (2 of 2) Class Main { public static void main(String[] args) { // Let's add up 10 dimes. System.out.println("10 dimes gives us:"); System.out.println(0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1); // Let's try do something a hundred times.. // but accidentally go forever int i = 100; do { // Make i one LARGER. Oops! i++; } while (i > 0); System.out.println("Done!"); System.out.println("i ended up with the value: " + i); System.out.println("It went down by: " + (100 - i)); } }

Number Representation Prediction // Let's add up 10 dimes. System.out.println("10 dimes gives us:"); System.out.println(0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1); // Let's try do something a hundred times.. // but accidentally go forever int i = 100; do { // Make i one LARGER. Oops! i++; } while (i > 0); • What will this print? • First 1.0 and then nothing because it runs forever. • First 1.0 and then some other value, because it won’t run forever. • First something OTHER than 1.0 and then nothing because it runs forever. • First something OTHER than 1.0 and then some other value, because it won’t forever. • None of these

Preface: Java Byte Code Java programs are compiled to a language called “byte code” that is then interpreted on a particular computer. Why? Byte code is hard for humans to write, read, and understand... but it’s easy to write a program that reads and executes it (compared to writing a program to directly read and execute Java source code). So, if you create a brand new type of computer tomorrow, and I want to run my Java code on it, I don’t have to write a program that works on your computer and knows how to execute Java; I just need to write a program that knows how to execute byte code. Java byte code is also designed to be compact so it’s cheap to transmit across the internet.

Problem: Java Byte Code Problem: When compiled to bytecode, i = 100 might be “push 100; store in variable 1”. The “opcode” for bipush (push a byte) is 1610. The opcode for istore_1 is 6010. Here’s a typical “hex” view of ~1/5th of the previous program’s byte code. Where is i = 100? a b b d c e: None of these.

Problem: Binary Byte Code Why would the same task (finding a particular snippet of code in a bytecode file) be much more difficult if the file were represented in binary? • Because we would have to translate all the opcodes and values to binary. • Because many bytecode files would have no binary representation. • Because the binary representation of the file would be much longer. • Because data like 1100100 (100 in base 2) might not show up as the sequence of numbers 1 1 0 0 1 0 0. • It wouldn’t be much more difficult.

Problem: Decimal Byte Code Why would the same task (finding a particular snippet of code in a bytecode file) be much more difficult if the file were represented in decimal? • Because we would have to translate all the opcodes and values to decimal. • Because many bytecode files would have no decimal representation. • Because the decimal representation of the file would be much longer. • Because data like 100 might not show up as the sequence of numbers 1 0 0. • It wouldn’t be much more difficult.

Learning Goals: In-Class By the end of this unit, you should be able to: • Critique the choice of a digital representation scheme—including describing its strengths, weaknesses, and flaws (such as imprecise representation or overflow) —for a given type of data and purpose, such as (1) fixed-width binary numbers using a two’s complement scheme for signed integer arithmetic in computers or (2) hexadecimal for human inspection of raw binary data. Discuss point of learning goals.

Next Lecture Learning Goals: Pre-Class By the start of class, you should be able to: • Use truth tables to establish or refute the validity of a rule of inference. • Given a rule of inference and propositional logic statements that correspond to the rule’s premises, apply the rule to infer a new statement implied by the original statements. Discuss point of learning goals.

Next Lecture Prerequisites Read Section 1.3 (Epp 3rd ed) or 2.3 (Epp 4th ed). Complete the open-book, untimed quiz on Vista that is due before next class.

Some Things to Try... (on your own if you have time, not required)

Problem: Weighty Numbers Problem: You have a balance scale and four weights. You may choose the mass of the weights, as long as they’re in whole units of grams. What’s the largest number n such that you can exactly measure every weight 0…n? ?g ?g ?g ?g

Problem: Representing Data Problem: Devise two different ways to represent each of the following with bits: • black-and-white images • text • the shape of your face

Representing Characters note: will come back to “hex American Standard Code or Information Interchange

CPSC 121: Models of Computation 2010 Winter Term 2